Monitoring, sensing and dosimetry of hazardous gas concentrations is extremely important to protect employees of industrial plants, the civilian populations surrounding chemical operations and military personnel. Numerous technologies have been developed for the estimation of the time weighted average gas concentration (TWA), and for the estimation of gas exposure levels. The most important methods are briefly described below:
1. Bubbling of a known quantity of air through an absorbing solution and measuring concentrations of its reaction product in the solution.
2. Passing a known quantity of gas through a column with a solid sorbent which either changes color or is desorbed in a subsequent step and the quantity of the desorbed gas is determined.
3. Adsorbtion of gas onto an activated carbon via a plastic material with large holes and measuring the quantity of adsorbed gas over a known period of time.
4. Absorbtion of gas into a solution and measuring the concentration of its reaction products continuously, e.g., using an electrochemical cell.
5. Measuring the change in the electrical properties of surfaces of solid-state devices after specific gases were adsorbed on them.
6. Measuring the length of a color-stain which develops in an open-end tube filled with a chromophore.
7. Using a diffusion-barrier based passive sampler with subsequent analysis by spectrometric, electrochemical, or chromatographic methods.
Commercially available gas dosimeters employing the techniques described above are generally designed for collecting the fugitive gas sample in the workplace, and then in a subsequent step, the dosimeter is analyzed by an appropriate analytical method or instrument. Typically, each dosimeter is designed for a particular gas, and a different analytical method is required for each gas. Moreover, commerically available gas monitoring devices which employ the above methods are very expensive. Therefore, there has been and continues to be a need for an economical gas monitoring technique which is suitable for a relatively wide range of gas monitoring problems.
A dosimeter with a built-in direct reading capability for giving a semi-quantitative assessment of the exposure level or with features which allow an auxiliary instrument to measure the exposure without further sample preparation would offer tremendous advantage over the above-noted dosimeters and methods in which the gas must be first collected and thereafter analyzed by an auxiliary method or instrument. While several such direct-reading devices have been proposed in the patent literature, they lack the degree of reliability and accuracy needed to make them generally acceptable and useful.
The present invention provides a passive colorimetric gas monitoring device which offers significant advantages over others which have been described in the patent literature or heretofore made available. Smith U.S. Pat. No. 3,681,027 described a passive colorimetric dosimeter for nitrogen dioxide which uses a solid impregnated with a diazotation mixture as a chromophore to detect nitrogen oxide. The dosimeter of Smith is not quantitative because the color reading is affected by the relative humidity and by the local superficial wind velocity. Moreover, the specific formulation used by Smith is not very stable and tends to decompose and forms color upon exposure to ozone, UV light and other reagents.
The passive dosimeters of Esch et al and of Moore (U.S. Pat. Nos. 4,205,043 and 4,472,353, respectively) offer advantages over that of Smith because they provide a reference window for color comparison, which can be used to compensate for the instability of the sensing chromophoric reagent. However, in both cases the color reading will be sensitive to the wind superficial velocity and to the relative humidity.
The dosimeter of McConnaughy (U.S. Pat. No. 4,478,792) offers some advantage over that of the previous two because it uses porous sheets impregnated with the chromophoric reagent to restrict the effect of wind velocity. However, it does not provide a way to compensate for the instability of the chromophoric reagent and the reading is still sensitive to the relative humidity. Other inventions have used diffusion barriers to overcome the effect of wind velocity, e.g., Cadoff and Hodgeron (Anal. Chem., 55 p. 2083-2085, 1983) and in several patents, e.g., Braun et al., U.S. Pat. No. 3,950,980; Goldsmith, U.S. Pat. No. 3,985,017; Kring U.S. Pat. Nos. 4,265,635 and 4,269,804.
The colorimetric gas dosimeters of the prior art have failed to appreciate or recognize that relative humidity almost always influences the color development of the sensing chromophoric reagent. Thus, one of the novelties of this invention is a method and apparatus which can eliminate the effect of the relative humidity as well as that of the air face velocity.